1. (Industrial Useful Field)
This invention relates to an electronically controlled automatic transmission for a vehicle.
2. (Prior Art)
A conventional automatic transmission for a vehicle has been so constructed that a driving range has been able to be selected to a driver's satisfaction by moving a manual shift lever 101 in a longitudinal direction of vehicle as illustrated by FIG. 15. Namely; P-range has been for a parking, R-range for a reverse, N-range for a neutral, D-range for an automatic speed change in first, second or third speed, 2-range for a second speed, and L-range for an automatic speed change in first or second speed, respectively. P-, R- and L-ranges have been selected by moving the manual shift lever 101 while pushing a push button 102. When a driver has intended to effect an engine brake or not to change a speed range during driving, he has selected 2-range or L-range by moving the manual shift lever 101.
In the above structure, however, it has frequently occured that D-range position or L-range position has been selected by passing an intented range when moving the manual shift lever 101 from L-range position to 2-range position or D-range position. This maloperation has arisen due to facts that the manual shift lever 101 has been moved in a straight line and a stopper of each drive range has been composed of a stopper mechanism provided with a weak pressing force. As a result, a driver has exhausted his nerves to operate the manual shift lever 101 correctly so that a disadvantage of worse operability has been involved in this structure.
The applicant of the present invention has already proposed the hydraulic speed change stage control unit for automatic transmission (Japanese Patent Application No. 62-49066). Such a control unit is so constructed that the driving range is fixed to a third speed for D-range, a second speed for 2-range and a first speed for L-range respectively, as shown by FIG. 8, in an emergency case where a failure occurs in an electronic control circuit 40 consisting of a microcomputer etc. and a power supply is turned off. On the other hand, in case where the electronic circuit 40 functions normally, the control unit is so constructed that a second speed solenoid valve S2 operates to shift the drive range down to the second speed when 2-position is selected during high speed driving at the third speed with D-range selected, and the second speed solenoid valve SL2 is kept operating in the high speed driving to prevent the driving range from being shifted down to the first speed when L-range is selected during driving with 2-range selected. This is because a speed change width between first and second speeds is generally larger than a speed change width between the second and third speeds, and this structure is indispensable for avoiding a breakage due to an excessive engine brake or an engine overrun.
In such a control unit, however, in case where the electronic control circuit 40 functions normally, a pressure oil passing through a passage 57 presses on an area C of a spool 64 of a 1st shift valve 60 before a solenoid valve S2 is energized to cause the pressure oil to press on an area A of a spool 66 of a 2nd shift valve 62, when L-range is selected during driving at third speed with D-range selected. Therefore, such a disadvantage instantaneously occurs that the drive range is changed to the first speed once and then to the second speed. In order to eliminate this disadvantage, a delay circuit must be installed in the passage 57 so that the hydraulic circuit becomes complicated to induce an increase in manufacturing cost.